JP3607249B2 - Thermoelectric conversion material and thermoelectric conversion element using the same - Google Patents
Thermoelectric conversion material and thermoelectric conversion element using the same Download PDFInfo
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Description
【０００１】
【発明の属する技術分野】
本発明は熱電変換材料および熱電変換素子に係わり、特にＦｅ−Ｖ−Ａｌ系の熱電変換材料およびそれを用いた熱電変換素子に関する。
【０００２】
【従来の技術】
近年、地球環境問題に対する意識の高揚から、フロンレス冷却機器であるペルチェ効果を利用した熱電冷却素子に関する関心が高まっている。また、同じく、二酸化炭素排出量を削減するために、未利用廃熱エネルギーを使った発電システムを提供する、ゼーベック効果を利用した熱電発電素子に関する関心が高まっている。
現在室温付近で利用されている熱電材料は、効率の高さから、Ｂｉ−Ｔｅ系の単結晶または多結晶体を使用したものが多い。この材料を用いて熱電素子化するためには、ｐ型、ｎ型両材料が必要となる。このうちｎ型材料には一般にＳｅが添加される。また、室温より高温で使用される熱電変換材料には、やはり効率の高さから、Ｐｂ−Ｔｅ系が用いられている。
ところで、これら素子に用いられているＳｅ（セレン），Ｐｂ（鉛），Ｔｅ（テルル）は人体にとって有毒有害であり、また地球環境問題の観点からも好ましくない。このため、これまでＢｉ−Ｔｅ系、Ｐｂ−Ｔｅ系材料に代わる無害な材料の検討がなされている。
【０００３】
現在検討されている材料のひとつに、Ｆｅ−Ｖ−Ａｌ系材料がある。Ｆｅ３ＡｌにおけるＦｅの１／３をＶで置換したＦｅ２ＶＡｌ合金はＬ２１型結晶構造（ホイスラー構造）を有し、半導体的な電気伝導の挙動を示すとともにＢｉ−Ｔｅ系材料に匹敵する高いゼーベック係数を室温で示すことが報告され注目を集めている（２０００年日本金属学会秋期大会講演概要ｐ．３６１）。さらに、Ｆｅ２ＶＡｌにおけるＡｌの一部をＳｉで置換した合金の出力因子は室温で５．４×１０−３Ｗ／ｍＫ２に達し、Ｂｉ−Ｔｅ系材料の４〜５−３Ｗ／ｍＫ２に匹敵する大きさであることが報告されている（日本金属学会誌 第６５巻 第７号（２００１）６５２−６５６）。
【０００４】
熱電変換材料の性能指数Ｚは、材料の熱起電力を示すゼーベック係数をα、導電率をσ、熱伝導率をκとした時、Ｚ＝α２σ／κで示され、α２σが前述した出力因子である。一般にＺの値が高いほど熱電変換材料としての性能が優れている。すなわち、熱電変換材料として応用するためには出力因子だけではなく、熱伝導率を小さくしてＺを上げることが必要である。
しかしながら、前述したＦｅ−Ｖ−Ａｌ合金は出力因子の観点ではＢｉ−Ｔｅ系材料に匹敵する高い値を有するものの、熱伝導度が１０倍程度高いため実用化に至っていない。
【０００５】
【発明が解決しようとする課題】
本発明はこのような観点からなされたものであり、Ｆｅ−Ｖ−Ａｌ合金を基に出力因子を出来るだけ損なうことなく熱伝導率を低下させ、性能指数Ｚの大きな熱電変換材料およびそれを用いた熱電変換素子を提供することを目的とする。
【０００６】
【課題を解決するための手段】
ホイスラー合金Ｆｅ２ＶＡｌは、バンド計算（Ｇ．Ｙ．Ｇｕｏ ｅｔ．ａｌ．，Ｊ．Ｐｈｙｓ．：Ｃｏｎｄｅｎｓ．Ｍａｔｔｅｒ．１０（１９９８）Ｌ１１９）によりその電子状態が詳しく調べられている。計算結果によると、フェルミ準位においてΓ点付近にＦｅの３ｄバンドからなる正孔ポケット、Ｘ点付近にＶの３ｄバンドからなる電子ポケットが形成されることがわかっており、フェルミ面のわずかな変化でキャリア濃度およびゼーベック係数が大きく変化することが推察される。本発明者らはＦｅ２ＶＡｌ近傍の組成におけるＦｅの一部をＣｒ、Ｍｎ、Ｃｏ、Ｎｉといった他の３ｄ遷移元素で置換した合金についてゼーベック係数の変化を調べたところ、置換元素がＦｅよりも周期律表で左側に位置するＣｒ、Ｍｎの場合にはゼーベック係数は正に、また、Ｆｅよりも周期律表で右側に位置するＣｏ、Ｎｉの場合には負になることを確認した。つまり、３ｄ電子数の制御でゼーベック係数の正負や絶対値を制御できることが示され、特にＭｎで置換した場合には１００μＶ／Ｋを超える正の大きなゼーベック係数が得られることが明らかになった。さらに、前記ＭｎでＦｅを置換した合金は、置換しない場合と比較して熱伝導率が低下することを見出し、その結果、高い性能指数を持つ熱電変換材料が実現され本発明に至ったものである。
【０００７】
すなわち、第１の本発明は、下記の組成式で表される組成を有することを特徴とする熱電変換材料である。
Ａｘ（Ｆｅ１―ａＤａ）ｙＶｚ（Ｅ１−ｂＧｂ）１００−ｘ―ｙ−ｚ
（式中、Ａは、ＭｎまたはＣｒの少なくとも一種、Ｄは、Ｔｉ，Ｃｏ，Ｎｉ，Ｃｕ，Ｚｎ，Ｚｒ，Ｎｂ，Ｍｏ，Ａｇ，Ｈｆ，Ｔａ，Ｗ，Ｙ，および希土類元素の群から選ばれる少なくとも一種の元素、ＥはＡｌまたはＳｉの少なくとも一種、Ｇは、Ｂ，Ｃ，Ｎ，Ｐ，Ｓ，Ｍｇ，Ｇａ，Ｇｅ，Ｓｎ，Ｉｎ，およびＢｉの群から選ばれる少なくとも一種の元素、ａ、ｂはそれぞれ０≦ａ≦０．２、０≦ｂ≦０．２、ｘ、ｙ、ｚはそれぞれ、２≦ｘ、３５≦ｘ＋ｙ≦６０、１５≦ｚ≦３５の数を表す。）
【０００８】
前記第１の本発明において、前記化学式のｘ＋ｙおよびｚは、それぞれ、２≦ｘ、４８≦ｘ＋ｙ≦５２、２５≦ｚ≦３３の数であることが望ましい。
前記第１の本発明の熱電変換材料において、Ｌ２１型結晶構造を有する結晶相が全結晶相および非晶質層の内の５０体積％以上を占める相であることが望ましい。このＬ２１型結晶構造を有する結晶相が５０体積％を下回った場合、十分な性能指数Ｚを有する材料が得られない。
【０００９】
第２の本発明は、電気的に接続されたｐ型熱電変換材料とｎ型熱電変換材料とを備えた熱電変換素子において、前記ｐ型熱電変換材料と前記ｎ型熱電変換材料のいずれか一方もしくは双方として、下記の組成式で表される材料を用いたことを特徴とする熱電変換素子である。
Ａｘ（Ｆｅ１―ａＤａ）ｙＶｚ（Ｅ１−ｂＧｂ）１００−ｘ―ｙ−ｚ
（式中、Ａは、ＭｎまたはＣｒの少なくとも一種、Ｄは、Ｔｉ，Ｃｏ，Ｎｉ，Ｃｕ，Ｚｎ，Ｚｒ，Ｎｂ，Ｍｏ，Ａｇ，Ｈｆ，Ｔａ，Ｗ，Ｙ，および希土類元素の群から選ばれる少なくとも一種の元素、ＥはＡｌまたはＳｉの少なくとも一種、Ｇは、Ｂ，Ｃ，Ｎ，Ｐ，Ｓ，Ｍｇ，Ｇａ，Ｇｅ，Ｓｎ，Ｉｎ，およびＢｉの群から選ばれる少なくとも一種の元素、ａ、ｂはそれぞれ０≦ａ≦０．２、０≦ｂ≦０．２、ｘ、ｙ、ｚはそれぞれ、２≦ｘ、３５≦ｘ＋ｙ≦６０、１５≦ｚ≦３５の数を表す。）
【００１０】
【発明の実施の形態】
まず、本発明の熱電変換材料について詳細に説明する。
［熱電変換材料］
本発明の熱電変換材料の一実施形態は、下記の組成式で表される組成を有するものである。
ＡｘＦｅｙＶｚＥ１００−ｘ―ｙ−ｚ
（式中、Ａは、ＭｎまたはＣｒの少なくとも一種、ＥはＡｌまたはＳｉの少なくとも一種、ｘ、ｙ、ｚはそれぞれ、２≦ｘ、３５≦ｘ＋ｙ≦６０、１５≦ｚ≦３５の数を表す。）
【００１１】
前記組成を有する材料の結晶形としては、Ｌ２１型結晶構造すなわちホイスラー型結晶構造や、α−Ｆｅ相などがあるが、本発明の熱電変換材料としては、これらの結晶形の内、Ｌ２１型結晶構造すなわちホイスラー型結晶構造を有する結晶相を全相の内の５０体積％以上とすることによって、より性能指数Ｚの大きな熱電変換材料が得られる。本発明の熱電変換材料を構成する他の相については特に制約を受けるものではなく、これらのいずれの相か、あるいは、非晶質相であっても良い。
【００１２】
以下、本発明を構成する各成分の配合理由および配合量の規定理由について述べる。
本発明の熱電変換材料には、ＭｎまたはＣｒが２原子％以上配合される。ＭｎまたはＣｒの配合量が２原子％未満であると前述した熱伝導率低下の効果が小さくなり好ましくない。より好ましいＭｎまたはＣｒの配合量は２〜５０原子％であり、さらに好ましくは５〜２５原子％である。
【００１３】
また、本発明の熱電変換材料は、Ａ元素すなわちＭｎまたはＣｒと、Ｆｅの総量が３５〜６０原子％の範囲で配合される。ＭｎまたはＣｒとＦｅの総量が３５原子％未満の場合、および６０原子％を超える場合には大きなゼーベック係数が得られない。より好ましいＭｎまたはＣｒとＦｅの総量は４０〜５５原子％、さらに好ましくは４２〜５２原子％である。
【００１４】
また、本発明の熱電変換材料はＶが、１５〜３５原子％の範囲で配合される。Ｖの配合量が１５原子％未満の場合、ホイスラー型結晶構造以外の結晶相が主相となってしまう恐れがあり、その結果、良好な熱電性能が得られない。また、Ｖの配合量が３５原子％を超えると、ゼーベック係数の低下が著しい。より好ましいＶ配合量の範囲は２０〜３０原子％、さらに好ましくは２２〜２８原子％である。
【００１５】
また、本発明の別の実施形態は、下記の組成式で表される組成を有するものである。
Ａｘ（Ｆｅ１―ａＤａ）ｙＶｚ（Ｅ１−ｂＧｂ）１００−ｘ―ｙ−ｚ
（式中、Ａは、ＭｎまたはＣｒの少なくとも一種、Ｄは、Ｔｉ，Ｃｏ，Ｎｉ，Ｃｕ，Ｚｎ，Ｚｒ，Ｎｂ，Ｍｏ，Ａｇ，Ｈｆ，Ｔａ，Ｗ，Ｙ，および希土類元素の群から選ばれる少なくとも一種の元素、ＥはＡｌまたはＳｉの少なくとも一種、Ｇは、Ｂ，Ｃ，Ｎ，Ｐ，Ｓ，Ｍｇ，Ｇａ，Ｇｅ，Ｓｎ，Ｉｎ，およびＢｉの群から選ばれる少なくとも一種の元素、ａ、ｂはそれぞれ０≦ａ≦０．２、０≦ｂ≦０．２、ｘ、ｙ、ｚはそれぞれ、２≦ｘ、３５≦ｘ＋ｙ≦６０、１５≦ｚ≦３５の数を表す。）
すなわち、先に述べた熱電変換材料において、Ｆｅの一部をＴｉ，Ｃｒ，Ｃｏ，Ｎｉ，Ｃｕ，Ｚｎ，Ｚｒ，Ｎｂ，Ｍｏ，Ａｇ，Ｈｆ，Ｔａ，Ｗ，Ｙ，および希土類元素の群から選ばれる少なくとも一種で置換することもできる。このような置換によって熱伝導率を更に低下させることができ、性能指数Ｚを高めることが可能である。ただし、過剰の置換はゼーベック係数の低下などによって逆にＺを低下させる恐れがあるため、置換する元素の量は、Ｆｅと置換元素量の総量に対して２０原子％以下とすることが好ましい。また、Ｆｅと置換元素量の総量に対して３原子％以上とすることが好ましく、これよりも少ないと置換することによる充分な効果が得られない。
【００１６】
また、本発明の熱電変換材料において、Ｄ元素すなわちＡｌまたはＳｉの一部をＢ，Ｃ，Ｎ，Ｐ，Ｓ，Ｍｇ，Ｇａ，Ｇｅ，Ｓｎ，Ｉｎ，Ｂｉの群から選ばれる少なくとも一種で置換することもできる。このような置換によって熱伝導率を更に低下することができ、性能指数Ｚを高めることが可能である。ただし、過剰の置換はゼーベック係数の低下などによって逆にＺを低下させる恐れがあるため、置換する元素の量は、Ｅ元素と置換元素量の総量に対して２０原子％以下とすることが好ましい。また、Ｆｅと置換元素量の総量に対して８原子％以上とすることが好ましく、これよりも少ないと置換することによる充分な効果が得られない。
【００１７】
以下、本発明の熱電変換材料成形体の製造例を説明する。
まず、上記組成式で示される所定量の各元素を含有する合金を、アーク溶解や高周波溶解などによって作製する。
この合金は、単ロール法、双ロール法、回転ディスク法、ガスアトマイズ法などの液体急冷法やメカニカルアロイング法などの固相反応を利用した方法で製造することができる。この液体急冷法やメカニカルアロイング法などによって合金を製造した場合、合金を構成する結晶相が微細化できることや、結晶相内への元素の固溶域を拡大することができるなどの効果があり、熱伝導度の低減、ゼーベック係数の増大などに有効である。また、この合金は、必要に応じて熱処理が施され、これによって合金を単相化したり、結晶粒子径を制御するなどして、さらに熱電特性を高めることも可能である。この工程で溶解、液体急冷、メカニカルアロイングおよび熱処理を実施する際の雰囲気はＡｒなどの不活性雰囲気中が好ましい。
【００１８】
次に、前記合金をボールミル、ブラウンミル、スタンプミルなどによって粉砕して合金粉末とした後、この合金粉末を焼結法、ホットプレス法、ＳＰＳ法などによって一体成型する。この工程で、一体成型を実施する際の雰囲気は、Ａｒなどの不活性雰囲気中が好ましい。
【００１９】
次いで、得られた成型体を例えば角柱状など所望の形状・寸法に機械加工して熱電変換材料成形体を製造することができる。
【００２０】
［熱電変換素子］
以下本発明の熱電変換素子について説明する。
本発明の熱電変換素子は、電気的に接続されたｐ型熱電変換材料成形体とｎ型熱電変換材料成形体とを備えた熱電変換素子において、前記ｐ型熱電変換材料と前記ｎ型熱電変換材料のいずれか一方もしくは双方として、下記の組成式で表される材料を用いたことを特徴とするものである。
Ａｘ（Ｆｅ１―ａＤａ）ｙＶｚ（Ｅ１−ｂＧｂ）１００−ｘ―ｙ−ｚ
（式中、Ａは、ＭｎまたはＣｒの少なくとも一種、Ｄは、Ｔｉ，Ｃｏ，Ｎｉ，Ｃｕ，Ｚｎ，Ｚｒ，Ｎｂ，Ｍｏ，Ａｇ，Ｈｆ，Ｔａ，Ｗ，Ｙ，および希土類元素の群から選ばれる少なくとも一種の元素、ＥはＡｌまたはＳｉの少なくとも一種、Ｇは、Ｂ，Ｃ，Ｎ，Ｐ，Ｓ，Ｍｇ，Ｇａ，Ｇｅ，Ｓｎ，Ｉｎ，およびＢｉの群から選ばれる少なくとも一種の元素、ａ、ｂはそれぞれ０≦ａ≦０．２、０≦ｂ≦０．２、ｘ、ｙ、ｚはそれぞれ、２≦ｘ、３５≦ｘ＋ｙ≦６０、１５≦ｚ≦３５の数を表す。）
ｐ型とｎ型のどちらか一方に本発明の熱電変換材料を用いる場合には、他方はＢｉ−Ｔｅ系材料など既知の材料を使用すればよい。
【００２１】
以下本発明に係る熱電変換素子の一態様を図１に示す。
図１において、１は本発明の熱電変換素子である。そして、例えば角柱状のｐ型熱電変換材料成形体２と、これも角柱状のｎ型熱電変換材料成形体３とを、並列に且つ離間するように配置し、これらの成形体の両端部を、例えば短冊状のアルミニウムなど導電材料からなる共通電極８によって電気的に直列に接続する。そして、両端部にある熱電変換材料成形体から、外部に取り出すための電極端子６，７を接続する。前記共通電極８の外側には、電気絶縁材料であり且つ熱伝導性材料からなる低温側熱伝導層４と、高温側熱伝導層５とを、覆設する。
この素子において、熱電変換材料成形体２，３と、共通電極８との接着接続は、公知の導電性接着剤によって行うことができる。また、共通電極８と低温側および高温側熱伝導層４，５との接着は、公知の有機接着剤もしくは無機接着剤を用いて行うことができる。
【００２２】
このような素子において、低温側熱伝導層４を低温度（Ｌ）にし、かつ高温側熱伝導層５を高温度（Ｈ）にして両熱伝導層に温度差を与えると、ｐ型半導体である熱電変換材料成形体２においては、正の電荷を持ったホールが低温度Ｌ側に、ｎ型半導体である熱電変換材料成形体３においては、負の電荷を持った電子が低温度側Ｌに移動する。その結果、電極端子６，７間に電位差が生じることになる。
一方、このような素子において、電極端子６を正極に、電極端子７を負極にして電圧を印加すると、前述と同様にホールおよび電子が移動して、個々の熱電変換材料成形体の両端に温度差が生じ、低温側熱伝導層４が低温に冷却され、一方高温側熱伝導層５が高温に加熱される。
このようにして本発明の熱電変換素子を、発電素子あるいは加熱・冷却素子として用いることができる。
【００２３】
上記実施の形態においては、複数の熱電変換材料を、線状に配列した例を示したが、熱電変換材料成形体を面状に配列することによってさらに熱電変換効率を向上させることもできる。
【００２４】
【実施例】
（実施例１）
所定量のＦｅ、Ｍｎ、Ｖ、Ａｌ原料を秤量してアーク溶解にて合金を製造した後、ボールミルを用いて４５μｍ以下に粉砕、９００℃で１時間ホットプレスすることにより外径１０ｍｍφ、厚み２ｍｍの成型体を得た。成型体の生成相をＸ線回折で調査したところ、ホイスラー型の結晶構造を有することを確認した。成型体の組成を表１に示した。成型体の熱拡散率をレーザーフラッシュ法、密度をアルキメデス法、比熱をＤＳＣ（示差走査熱量計）法でそれぞれ測定し、それらの結果から熱伝導率κを求めたところ、３００Ｋで４．５Ｗ／ｍＫであった。また、前記成型体を針状に切り出してゼーベック係数αを測定したところ、３００Ｋで１１５μＶ／Ｋであった。さらに、前記針状の成型体の電気抵抗率ρを４端子法にて測定した結果、３００Ｋで０．９２ｍΩｃｍであった。これらの結果から性能指数Ｚ（Ｚ＝α２／ρκ）を求めたところ、３．１９×１０−４Ｋ−１であった。
【００２５】
（実施例２〜７、比較例１）
実施例１と同様の方法で合金を製造、実施例１と同様にボールミル粉砕、ホットプレスすることにより成型体を得た。成型体の生成相をＸ線回折で調査したところ、いずれもホイスラー型の結晶構造を有することを確認した。実施例２〜５、比較例１の各成型体の組成を表１に示した。また、実施例１と同様の方法で求めた３００Ｋでの性能指数Ｚの値を表１に併記した。
実施例１〜５および比較例１から、Ｆｅの一部をＭｎで置換した本発明の組成の方が熱伝導率が小さく、その結果、性能指数Ｚの値が高いことがわかる。
【００２６】
（実施例８）
所定量のＦｅ、Ｍｎ、Ｖ、Ａｌ原料を秤量してアーク溶解にて合金を製造した後、前記合金をＡｒ雰囲気中で溶解し、４０ｍ／ｓの周速度で回転する直径３００ｍｍの銅製ロールに射出する液体急冷法により急冷薄帯を作製した。次いで、この急冷薄帯をボールミルを用いて４５μｍ以下に粉砕、８５０℃で３０分間ホットプレスすることにより外径１０ｍｍφ、厚み２ｍｍの成型体を得た。成型体の生成相をＸ線回折で調査したところ、ホイスラー型の結晶構造を有することを確認した。成型体の組成を表１に示した。成型体の熱拡散率をレーザーフラッシュ法、密度をアルキメデス法、比熱をＤＳＣ（示差走査熱量計）法でそれぞれ測定し、それらの結果から熱伝導率κを求めたところ、３００Ｋで３．８Ｗ／ｍＫであった。また、前記成型体を針状に切り出してゼーベック係数αを測定したところ、３００Ｋで１１８μＶ／Ｋであった。さらに、前記針状の成型体の電気抵抗率ρを４端子法にて測定した結果、３００Ｋで０．９５ｍΩｃｍであった。これらの結果から性能指数Ｚ（Ｚ＝α２／ρκ）を求めたところ、３．８６×１０−４Ｋ−１であった。
【００２７】
（実施例９〜１２、比較例２）
実施例８と同様の方法で急冷薄帯を作製、実施例８と同様にボールミル粉砕、ホットプレスすることにより成型体を得た。成型体の生成相をＸ線回折で調査したところ、いずれもホイスラー型の結晶構造を有することを確認した。実施例９〜１２、比較例２の各成型体の組成を表１に示した。また、実施例１と同様の方法で求めた３００Ｋでの性能指数Ｚの値を表１に併記した。
実施例８〜１２および比較例２から、Ｆｅの一部をＭｎで置換した本発明の組成の方が熱伝導率が小さく、その結果、性能指数Ｚの値が高いことがわかる。
【００２８】
【表１】
また、実施例３のＷに代えて、Ｚｎ、Ａｇ、Ｈｆ又はＹでＦｅの一部を置換した場合にも、Ｆｅを置換しない場合に比べて熱伝導率が小さく、その結果、性能指数Ｚの値が高かった。
また、実施例４のＧａに代えて、Ｐ，Ｓ，Ｍｇ，Ｇｅ，Ｓｎ，Ｉｎ又はＢｉでＡｌの一部を置換した場合にも、Ｆｅを置換しない場合に比べて熱伝導率が小さく、その結果、性能指数Ｚの値が高かった。
【００３０】
【発明の効果】
以上説明したように、本発明によればＦｅ−Ｖ−Ａｌ合金のＦｅの一部をＭｎまたはＣｒで置換した組成の合金を採用することにより、熱伝導率が小さく、性能指数Ｚの大きな熱電変換材料を提供でき、これによって優れた性能の熱電変換素子を提供することが可能となった。本発明の熱電変換材料および熱電変換素子は従来より知られたＢｉ−Ｔｅ系材料と比較して毒性が小さいため地球環境問題の観点からも好ましく、工業的価値は大なるものがある。
【図面の簡単な説明】
【図１】本発明の熱電変換素子の１例を示す概略図
【符号の説明】
１・・・熱電変換素子
２・・・ｐ型熱電変換材料成形体
３・・・ｎ型熱電変換材料成形体
４・・・低温度側熱伝導層
５・・・高温度側熱伝導層
６，７・・・電極端子
８・・・共通電極[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a thermoelectric conversion material and a thermoelectric conversion element, and more particularly to an Fe—V—Al-based thermoelectric conversion material and a thermoelectric conversion element using the same.
[0002]
[Prior art]
In recent years, interest in the thermoelectric cooling element using the Peltier effect, which is a chlorofluorocarbon-free cooling device, has increased due to the heightened awareness of global environmental problems. Similarly, in order to reduce carbon dioxide emissions, there is an increasing interest in thermoelectric power generation elements using the Seebeck effect that provide power generation systems using unused waste heat energy.
Many thermoelectric materials currently used near room temperature use Bi-Te single crystals or polycrystals because of their high efficiency. In order to make a thermoelectric element using this material, both p-type and n-type materials are required. Of these, Se is generally added to n-type materials. Also, Pb—Te system is used for thermoelectric conversion materials used at a temperature higher than room temperature because of its high efficiency.
By the way, Se (selenium), Pb (lead), and Te (tellurium) used in these elements are toxic and harmful to the human body and are not preferable from the viewpoint of global environmental problems. For this reason, harmless materials that replace Bi—Te and Pb—Te materials have been studied so far.
[0003]
One of the materials currently being studied is an Fe-V-Al-based material. An Fe2VAl alloy in which 1/3 of Fe in Fe3Al is replaced with V has an L2 type 1 crystal structure (Heusler structure), exhibits a semiconductor-like electrical conduction behavior, and has a high Seebeck coefficient comparable to Bi-Te materials. It is reported to show at room temperature and attracts attention (Outline of the 2000 Annual Meeting of the Japan Institute of Metals p.361). Furthermore, the power factor of the alloy in which part of Al in Fe2VAl is replaced with Si reaches 5.4 × 10 −3 W / mK 2 at room temperature, and reaches 4 to 5 −3 W / mK 2 of Bi—Te based material. It is reported that the size is comparable (The Japan Institute of Metals, Vol. 65, No. 7 (2001) 652-656).
[0004]
The performance index Z of the thermoelectric conversion material is expressed as Z = α 2 σ / κ, where α is the Seebeck coefficient indicating the thermoelectromotive force of the material, σ is the conductivity, and κ is the thermal conductivity, and α 2 σ is This is the output factor described above. In general, the higher the value of Z, the better the performance as a thermoelectric conversion material. That is, in order to apply as a thermoelectric conversion material, it is necessary to increase not only the output factor but also the thermal conductivity to increase Z.
However, although the aforementioned Fe-V-Al alloy has a high value comparable to the Bi-Te material in terms of output factor, it has not been put into practical use because its thermal conductivity is about 10 times higher.
[0005]
[Problems to be solved by the invention]
The present invention has been made from such a viewpoint, and based on the Fe-V-Al alloy, the thermal conductivity is reduced without losing the output factor as much as possible, and the thermoelectric conversion material having a large figure of merit Z is used. An object of the present invention is to provide a thermoelectric conversion element.
[0006]
[Means for Solving the Problems]
The electronic state of the Heusler alloy Fe2VAl has been investigated in detail by band calculation (G. Guo et. Al., J. Phys .: Condens. Matter. 10 (1998) L119). According to the calculation results, it is known that a hole pocket composed of Fe 3d band is formed near the Γ point in the Fermi level, and an electron pocket composed of 3d band of V is formed near the X point. It is inferred that the carrier concentration and Seebeck coefficient change greatly with the change. The present inventors examined the change in the Seebeck coefficient for an alloy in which a part of Fe in the composition near Fe2VAl was replaced with another 3d transition element such as Cr, Mn, Co, and Ni. It was confirmed that the Seebeck coefficient was positive in the case of Cr and Mn located on the left side in the table and negative in the case of Co and Ni located on the right side of the periodic table rather than Fe. In other words, it was shown that the positive / negative and absolute values of the Seebeck coefficient can be controlled by controlling the number of 3d electrons, and in particular, when it was substituted with Mn, it became clear that a large positive Seebeck coefficient exceeding 100 μV / K could be obtained. Furthermore, it has been found that the alloy in which Fe is substituted with Mn has a lower thermal conductivity as compared with the case where Fe is not substituted, and as a result, a thermoelectric conversion material having a high figure of merit has been realized and the present invention has been achieved. is there.
[0007]
That is, the first aspect of the present invention is a thermoelectric conversion material having a composition represented by the following composition formula.
A x (Fe 1-a D a) y V z (E 1-b G b) 100-x-y-z
(Wherein A is at least one of Mn or Cr, D is selected from the group of Ti, Co, Ni, Cu, Zn, Zr, Nb, Mo, Ag, Hf, Ta, W, Y, and rare earth elements) At least one element selected from the group consisting of B, C, N, P, S, Mg, Ga, Ge, Sn, In, and Bi, a, b are 0 ≦ a ≦ 0.2, 0 ≦ b ≦ 0.2, x, y, and z are numbers of 2 ≦ x, 35 ≦ x + y ≦ 60, and 15 ≦ z ≦ 35, respectively.
[0008]
In the first aspect of the present invention, x + y and z in the chemical formula are preferably numbers of 2 ≦ x, 48 ≦ x + y ≦ 52, and 25 ≦ z ≦ 33, respectively.
In the thermoelectric conversion material of the first aspect of the present invention, it is desirable that the crystal phase having the L2 type 1 crystal structure is a phase occupying 50% by volume or more of the total crystal phase and the amorphous layer. When the crystal phase having this L2 type 1 crystal structure is less than 50% by volume, a material having a sufficient figure of merit Z cannot be obtained.
[0009]
2nd this invention is a thermoelectric conversion element provided with the p-type thermoelectric conversion material and n-type thermoelectric conversion material which were electrically connected, Either one of the said p-type thermoelectric conversion material and the said n-type thermoelectric conversion material Or it is a thermoelectric conversion element characterized by using the material represented by the following compositional formula as both.
A x (Fe 1-a D a) y V z (E 1-b G b) 100-x-y-z
(Wherein A is at least one of Mn or Cr, D is selected from the group of Ti, Co, Ni, Cu, Zn, Zr, Nb, Mo, Ag, Hf, Ta, W, Y, and rare earth elements) At least one element selected from the group consisting of B, C, N, P, S, Mg, Ga, Ge, Sn, In, and Bi, a, b are 0 ≦ a ≦ 0.2, 0 ≦ b ≦ 0.2, x, y, and z are numbers of 2 ≦ x, 35 ≦ x + y ≦ 60, and 15 ≦ z ≦ 35, respectively.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
First, the thermoelectric conversion material of the present invention will be described in detail.
[Thermoelectric conversion material]
One embodiment of the thermoelectric conversion material of the present invention has a composition represented by the following composition formula.
A x Fe y V z E 100-xyz
(In the formula, A represents at least one of Mn or Cr, E represents at least one of Al or Si, and x, y, and z represent numbers of 2 ≦ x, 35 ≦ x + y ≦ 60, and 15 ≦ z ≦ 35, respectively. .)
[0011]
Examples of the crystal form of the material having the above composition include an L2 type 1 crystal structure, that is, a Heusler type crystal structure, an α-Fe phase, and the like. The thermoelectric conversion material of the present invention includes L2 1 A thermoelectric conversion material having a larger figure of merit Z can be obtained by setting the crystal phase having a type crystal structure, that is, a Heusler type crystal structure, to 50% by volume or more of all phases. The other phases constituting the thermoelectric conversion material of the present invention are not particularly limited, and any of these phases or an amorphous phase may be used.
[0012]
Hereinafter, the reason for blending each component constituting the present invention and the reason for defining the blending amount will be described.
In the thermoelectric conversion material of the present invention, 2 atomic% or more of Mn or Cr is blended. If the blending amount of Mn or Cr is less than 2 atomic%, the above-described effect of decreasing the thermal conductivity is reduced, which is not preferable. A more preferable amount of Mn or Cr is 2 to 50 atomic%, and further preferably 5 to 25 atomic%.
[0013]
Moreover, the thermoelectric conversion material of this invention is mix | blended in the range whose element A, ie, Mn or Cr, and the total amount of Fe are 35-60 atomic%. When the total amount of Mn or Cr and Fe is less than 35 atomic% and exceeds 60 atomic%, a large Seebeck coefficient cannot be obtained. More preferably, the total amount of Mn or Cr and Fe is 40 to 55 atomic%, more preferably 42 to 52 atomic%.
[0014]
Moreover, V is mix | blended in the range of 15-35 atomic% in the thermoelectric conversion material of this invention. When the compounding amount of V is less than 15 atomic%, a crystal phase other than the Heusler type crystal structure may become a main phase, and as a result, good thermoelectric performance cannot be obtained. Moreover, when the compounding amount of V exceeds 35 atomic%, the Seebeck coefficient is remarkably lowered. A more preferable range of the V content is 20 to 30 atomic%, and further preferably 22 to 28 atomic%.
[0015]
Another embodiment of the present invention has a composition represented by the following composition formula.
A x (Fe 1-a D a) y V z (E 1-b G b) 100-x-y-z
(Wherein A is at least one of Mn or Cr, D is selected from the group of Ti, Co, Ni, Cu, Zn, Zr, Nb, Mo, Ag, Hf, Ta, W, Y, and rare earth elements) At least one element selected from the group consisting of B, C, N, P, S, Mg, Ga, Ge, Sn, In, and Bi, a, b are 0 ≦ a ≦ 0.2, 0 ≦ b ≦ 0.2, x, y, and z are numbers of 2 ≦ x, 35 ≦ x + y ≦ 60, and 15 ≦ z ≦ 35, respectively.
That is, in the thermoelectric conversion material described above, a part of Fe is derived from the group of Ti, Cr, Co, Ni, Cu, Zn, Zr, Nb, Mo, Ag, Hf, Ta, W, Y, and rare earth elements. It can also be substituted with at least one selected. Such substitution can further lower the thermal conductivity and increase the figure of merit Z. However, since excessive substitution may lower Z due to a decrease in Seebeck coefficient or the like, the amount of the element to be substituted is preferably 20 atomic% or less with respect to the total amount of Fe and the amount of substitution element. Moreover, it is preferable to set it as 3 atomic% or more with respect to the total amount of Fe and the amount of substitution elements, and when less than this, sufficient effect by substitution will not be acquired.
[0016]
In the thermoelectric conversion material of the present invention, a part of the D element, that is, Al or Si, is replaced with at least one selected from the group of B, C, N, P, S, Mg, Ga, Ge, Sn, In, and Bi. You can also Such substitution can further lower the thermal conductivity and increase the figure of merit Z. However, since excessive substitution may lower Z due to a decrease in Seebeck coefficient or the like, the amount of elements to be substituted is preferably 20 atomic% or less with respect to the total amount of element E and the amount of substitution elements. . Moreover, it is preferable to set it as 8 atomic% or more with respect to the total amount of Fe and the amount of substitution elements, and when less than this, sufficient effect by substitution will not be acquired.
[0017]
Hereinafter, the manufacture example of the thermoelectric conversion material molded object of this invention is demonstrated.
First, an alloy containing a predetermined amount of each element represented by the above composition formula is produced by arc melting, high frequency melting or the like.
This alloy can be produced by a method using a solid phase reaction such as a liquid quenching method such as a single roll method, a twin roll method, a rotating disk method, a gas atomizing method, or a mechanical alloying method. When an alloy is manufactured by this liquid quenching method or mechanical alloying method, there is an effect that the crystal phase constituting the alloy can be refined and the solid solution region of the element into the crystal phase can be expanded. It is effective for reducing thermal conductivity and increasing Seebeck coefficient. In addition, the alloy can be heat-treated as necessary, and the thermoelectric properties can be further improved by making the alloy into a single phase or controlling the crystal particle diameter. In this step, the atmosphere for carrying out dissolution, liquid quenching, mechanical alloying and heat treatment is preferably an inert atmosphere such as Ar.
[0018]
Next, the alloy is pulverized by a ball mill, a brown mill, a stamp mill or the like to form an alloy powder, and then the alloy powder is integrally formed by a sintering method, a hot press method, an SPS method, or the like. In this step, the atmosphere for carrying out the integral molding is preferably an inert atmosphere such as Ar.
[0019]
Next, the obtained molded body can be machined into a desired shape and size such as a prismatic shape to produce a thermoelectric conversion material molded body.
[0020]
[Thermoelectric conversion element]
The thermoelectric conversion element of the present invention will be described below.
The thermoelectric conversion element of the present invention is a thermoelectric conversion element comprising an electrically connected p-type thermoelectric conversion material molded body and an n-type thermoelectric conversion material molded body, wherein the p-type thermoelectric conversion material and the n-type thermoelectric conversion are provided. As one or both of the materials, a material represented by the following composition formula is used.
A x (Fe 1-a D a) y V z (E 1-b G b) 100-x-y-z
(Wherein A is at least one of Mn or Cr, D is selected from the group of Ti, Co, Ni, Cu, Zn, Zr, Nb, Mo, Ag, Hf, Ta, W, Y, and rare earth elements) At least one element selected from the group consisting of B, C, N, P, S, Mg, Ga, Ge, Sn, In, and Bi, a, b are 0 ≦ a ≦ 0.2, 0 ≦ b ≦ 0.2, x, y, and z are numbers of 2 ≦ x, 35 ≦ x + y ≦ 60, and 15 ≦ z ≦ 35, respectively.
When the thermoelectric conversion material of the present invention is used for either the p-type or the n-type, a known material such as a Bi—Te-based material may be used for the other.
[0021]
An embodiment of a thermoelectric conversion element according to the present invention is shown in FIG.
In FIG. 1, 1 is a thermoelectric conversion element of the present invention. For example, a prismatic p-type thermoelectric conversion material molded body 2 and a prismatic n-type thermoelectric conversion material molded
In this element, the adhesive connection between the thermoelectric conversion material molded
[0022]
In such an element, when the low temperature side heat conductive layer 4 is set to a low temperature (L) and the high temperature side heat conductive layer 5 is set to a high temperature (H) to give a temperature difference between the two heat conductive layers, a p-type semiconductor is formed. In a certain thermoelectric conversion material molded body 2, positively charged holes are on the low temperature side L, and in a thermoelectric conversion material molded
On the other hand, in such an element, when a voltage is applied with the
Thus, the thermoelectric conversion element of the present invention can be used as a power generation element or a heating / cooling element.
[0023]
In the said embodiment, although the example which arranged the some thermoelectric conversion material in the linear form was shown, thermoelectric conversion efficiency can also be improved further by arranging the thermoelectric conversion material molded object in planar shape.
[0024]
【Example】
Example 1
A predetermined amount of Fe, Mn, V, and Al raw materials are weighed and an alloy is produced by arc melting, then pulverized to 45 μm or less using a ball mill, and hot-pressed at 900 ° C. for 1 hour for an outer diameter of 10 mmφ and a thickness of 2 mm A molded body was obtained. When the formed phase of the molded body was examined by X-ray diffraction, it was confirmed that it had a Heusler type crystal structure. The composition of the molded body is shown in Table 1. The thermal diffusivity of the molded body was measured by the laser flash method, the density was measured by the Archimedes method, the specific heat was measured by the DSC (Differential Scanning Calorimetry) method, and the thermal conductivity κ was determined from these results. mK. Further, when the molded body was cut out in a needle shape and the Seebeck coefficient α was measured, it was 115 μV / K at 300K. Furthermore, as a result of measuring the electrical resistivity ρ of the needle-shaped molded body by the 4-terminal method, it was 0.92 mΩcm at 300K. The figure of merit Z (Z = α 2 / ρκ) was determined from these results and found to be 3.19 × 10 −4 K −1 .
[0025]
(Examples 2-7, Comparative Example 1)
An alloy was produced in the same manner as in Example 1, and a molded body was obtained by ball milling and hot pressing in the same manner as in Example 1. When the formed phase of the molded body was examined by X-ray diffraction, it was confirmed that all had a Heusler type crystal structure. The compositions of the molded articles of Examples 2 to 5 and Comparative Example 1 are shown in Table 1. Table 1 also shows the value of the figure of merit Z at 300 K obtained by the same method as in Example 1.
From Examples 1 to 5 and Comparative Example 1, it can be seen that the composition of the present invention in which part of Fe is substituted with Mn has a smaller thermal conductivity, and as a result, the value of the figure of merit Z is higher.
[0026]
(Example 8)
After a predetermined amount of Fe, Mn, V, and Al raw materials are weighed and an alloy is manufactured by arc melting, the alloy is melted in an Ar atmosphere, and is turned into a copper roll having a diameter of 300 mm that rotates at a peripheral speed of 40 m / s. Quenched ribbons were prepared by the liquid quenching method. Next, the quenched ribbon was pulverized to 45 μm or less using a ball mill and hot pressed at 850 ° C. for 30 minutes to obtain a molded body having an outer diameter of 10 mmφ and a thickness of 2 mm. When the formed phase of the molded body was examined by X-ray diffraction, it was confirmed that it had a Heusler type crystal structure. The composition of the molded body is shown in Table 1. The thermal diffusivity of the molded body was measured by the laser flash method, the density was measured by the Archimedes method, the specific heat was measured by the DSC (differential scanning calorimeter) method, and the thermal conductivity κ was determined from these results. mK. Further, when the molded body was cut out in a needle shape and the Seebeck coefficient α was measured, it was 118 μV / K at 300K. Furthermore, as a result of measuring the electrical resistivity ρ of the needle-like molded body by the 4-terminal method, it was 0.95 mΩcm at 300K. When the figure of merit Z (Z = α 2 / ρκ) was determined from these results, it was 3.86 × 10 −4 K −1 .
[0027]
(Examples 9 to 12, Comparative Example 2)
A quenched ribbon was prepared in the same manner as in Example 8, and a molded body was obtained by ball milling and hot pressing in the same manner as in Example 8. When the formed phase of the molded body was examined by X-ray diffraction, it was confirmed that all had a Heusler type crystal structure. The compositions of the molded articles of Examples 9 to 12 and Comparative Example 2 are shown in Table 1. Table 1 also shows the value of the figure of merit Z at 300 K obtained by the same method as in Example 1.
From Examples 8 to 12 and Comparative Example 2, it can be seen that the composition of the present invention in which part of Fe is substituted with Mn has a lower thermal conductivity, and as a result, the value of the figure of merit Z is higher.
[0028]
[Table 1]
In addition, even when a part of Fe is substituted with Zn, Ag, Hf, or Y instead of W in Example 3, the thermal conductivity is smaller than when Fe is not substituted, and as a result, the figure of merit Z The value of was high.
Further, in place of Ga in Example 4, even when a part of Al is substituted with P, S, Mg, Ge, Sn, In, or Bi, the thermal conductivity is small compared to the case where Fe is not substituted, As a result, the value of the figure of merit Z was high.
[0030]
【The invention's effect】
As described above, according to the present invention, by adopting an alloy having a composition in which a part of Fe of an Fe-V-Al alloy is replaced with Mn or Cr, a thermoelectric power having a small thermal conductivity and a large figure of merit Z is obtained. A conversion material can be provided, which makes it possible to provide a thermoelectric conversion element with excellent performance. The thermoelectric conversion material and the thermoelectric conversion element of the present invention are preferable from the viewpoint of global environmental problems because they are less toxic than the conventionally known Bi-Te materials, and have a large industrial value.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing an example of a thermoelectric conversion element of the present invention.
DESCRIPTION OF
Claims (4)
Ａｘ（Ｆｅ１―ａＤａ）ｙＶｚ（Ｅ１−ｂＧｂ）１００−ｘ―ｙ−ｚ
（式中、Ａは、ＭｎまたはＣｒの少なくとも一種、Ｄは、Ｔｉ，Ｃｏ，Ｎｉ，Ｃｕ，Ｚｎ，Ｚｒ，Ｎｂ，Ｍｏ，Ａｇ，Ｈｆ，Ｔａ，Ｗ，Ｙ，および希土類元素の群から選ばれる少なくとも一種の元素、ＥはＡｌまたはＳｉの少なくとも一種、Ｇは、Ｂ，Ｃ，Ｎ，Ｐ，Ｓ，Ｍｇ，Ｇａ，Ｇｅ，Ｓｎ，Ｉｎ，およびＢｉの群から選ばれる少なくとも一種の元素、ａ、ｂはそれぞれ０≦ａ≦０．２、０≦ｂ≦０．２、ｘ、ｙ、ｚはそれぞれ、２≦ｘ、３５≦ｘ＋ｙ≦６０、１５≦ｚ≦３５の数を表す。）A thermoelectric conversion material having a composition represented by the following composition formula:
A x (Fe 1-a D a) y V z (E 1-b G b) 100-x-y-z
(Wherein A is at least one of Mn or Cr, D is selected from the group of Ti, Co, Ni, Cu, Zn, Zr, Nb, Mo, Ag, Hf, Ta, W, Y, and rare earth elements) At least one element selected from the group consisting of B, C, N, P, S, Mg, Ga, Ge, Sn, In, and Bi, a, b are 0 ≦ a ≦ 0.2, 0 ≦ b ≦ 0.2, x, y, and z are numbers of 2 ≦ x, 35 ≦ x + y ≦ 60, and 15 ≦ z ≦ 35, respectively.
Ａｘ（Ｆｅ１―ａＤａ）ｙＶｚ（Ｅ１−ｂＧｂ）１００−ｘ―ｙ−ｚ
（式中、Ａは、ＭｎまたはＣｒの少なくとも一種、Ｄは、Ｔｉ，Ｃｏ，Ｎｉ，Ｃｕ，Ｚｎ，Ｚｒ，Ｎｂ，Ｍｏ，Ａｇ，Ｈｆ，Ｔａ，Ｗ，Ｙ，および希土類元素の群から選ばれる少なくとも一種の元素、ＥはＡｌまたはＳｉの少なくとも一種、Ｇは、Ｂ，Ｃ，Ｎ，Ｐ，Ｓ，Ｍｇ，Ｇａ，Ｇｅ，Ｓｎ，Ｉｎ，およびＢｉの群から選ばれる少なくとも一種の元素、ａ、ｂはそれぞれ０≦ａ≦０．２、０≦ｂ≦０．２、ｘ、ｙ、ｚはそれぞれ、２≦ｘ、３５≦ｘ＋ｙ≦６０、１５≦ｚ≦３５の数を表す。）In a thermoelectric conversion element comprising an electrically connected p-type thermoelectric conversion material and an n-type thermoelectric conversion material, the following composition is used as one or both of the p-type thermoelectric conversion material and the n-type thermoelectric conversion material: The thermoelectric conversion element characterized by using the material represented by Formula.
A x (Fe 1-a D a) y V z (E 1-b G b) 100-x-y-z
(Wherein A is at least one of Mn or Cr, D is selected from the group of Ti, Co, Ni, Cu, Zn, Zr, Nb, Mo, Ag, Hf, Ta, W, Y, and rare earth elements) At least one element selected from the group consisting of B, C, N, P, S, Mg, Ga, Ge, Sn, In, and Bi, a, b are 0 ≦ a ≦ 0.2, 0 ≦ b ≦ 0.2, x, y, and z are numbers of 2 ≦ x, 35 ≦ x + y ≦ 60, and 15 ≦ z ≦ 35, respectively.
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| JP2004119648A (en) * | 2002-09-26 | 2004-04-15 | Toshiba Corp | p-TYPE THERMOELECTRIC CONVERSION MATERIAL AND THERMOELECTRIC CONVERSION ELEMENT USING IT |
| JP4521215B2 (en) * | 2004-03-31 | 2010-08-11 | 株式会社東芝 | Thermoelectric conversion material and thermoelectric conversion element |
| EP2662466A3 (en) | 2004-04-21 | 2014-08-06 | Showa Denko K.K. | Process for producing thermoelectric semiconductor alloy, thermoelectric conversion module, thermoelectric power generating device, rare earth alloy, producing process thereof, thermoelectric conversion material, and thermoelectric conversion system using filled skutterudite based alloy |
| JP2006203186A (en) * | 2004-12-24 | 2006-08-03 | Showa Denko Kk | Method of producing thermoelectric semiconductor alloy, thermoelectric conversion module, and thermoelectric power generation device |
| JP4809691B2 (en) | 2006-02-24 | 2011-11-09 | ヤンマー株式会社 | Iron alloy thermoelectric material |
| JP5303721B2 (en) * | 2006-03-17 | 2013-10-02 | 国立大学法人 名古屋工業大学 | Thermoelectric conversion material |
| JP4745183B2 (en) * | 2006-09-29 | 2011-08-10 | 株式会社東芝 | Thermoelectric conversion material and thermoelectric conversion module using the same |
| JP4858976B2 (en) * | 2007-01-31 | 2012-01-18 | 独立行政法人産業技術総合研究所 | Composite thermoelectric conversion material |
| JP5723591B2 (en) | 2010-12-28 | 2015-05-27 | 株式会社日立製作所 | Thermoelectric conversion materials and electric field applied thermoelectric conversion elements |
| JP7209167B2 (en) * | 2017-05-08 | 2023-01-20 | パナソニックＩｐマネジメント株式会社 | Jintle phase thermoelectric conversion material |
| KR102364931B1 (en) * | 2018-01-19 | 2022-02-17 | 주식회사 엘지화학 | Semiconductor compound and thermoelectric element including the same |
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